The use of various boron-containing compounds as extreme pressure and anti-wear additives for lubricating oils has been suggested in the prior art.
There continues to be a need for lubricants of sufficient lubricity to provide lubricating between the bearing surfaces of moving metal components where the bearing surfaces are subjected to large forces at the point of contact. These forces and the accompanying friction result in the generation of heat which elevates the metal temperatures. If the pressure is sufficiently high and the relative motion between surfaces sufficient, very high metal temperatures can result. Thus, in extreme cases, the contacting metal surfaces may actually weld one to the other. With less extremes of pressure, inadequate lubrication between surfaces is manifested in accelerated wear, scuffing, etc.
In addition to anti-wear and extreme pressure properties, lubricants for use in limited slip differentials require unusual frictional properties. Limited slip differentials are unlike conventional differentials in that they have the ability to move a vehicle when only one wheel has traction. This advantage is accomplished through an internal clutch that restricts conventional differential action and tends to make the left and right rear axle shafts turn as one. There are a variety of clutch arrangements employed to accomplish this result but most rely on the force of friction to restrict the free turning of the axle shaft. Since the clutches are continuously contacted by the differential lubricant, the restricting force depends on the coefficient of friction imparted to the engaging parts by said lubricant. The coefficient of friction must be compromised. On the other hand, it must be high enough to allow the clutch plates to seize, thus allowing the vehicle to move in instances when one rear wheel has low traction and, on the other hand, it must be sufficiently low to allow the clutch plates to slip and accomodate conventional differential action such as occurs when the vehicle turns corners. The ability of a lubricant to impart a coefficient of friction within this fairly wide effective range is referred to as effectiveness.
Another important aspect of frictional properties relates to the chatter or noise caused by the differential action of clutches. Chatter is generated by a stick-slip action of the engaging parts of the clutches. Differential manufacturers have found that stick-slip is primarily the result of the static coefficient of friction being greater than the dynamic coefficient of friction. That is, when the ratio of the static coefficient of friction to the dynamic coefficient of friction is greater than one.
Many diverse additives for enhancing anti-wear, extreme pressure and friction modifying properties are well known in the prior art. Examples of such additives are chlorinated waxes, alkyl polysulfides, alkyl phosphites, alkaryl phosphates, metal dithiophosphates, sulfurized sperm oil, sulfurized olefins, alkali metal borates, and the like. However, a need still exists for materials which can provide these properties and combinations of these properties.
U.S. Pat. No. 4,512,903 describes the preparation of amides from mono- or polyhydroxy-substituted aliphatic monocarboxylic acids and primary or secondary amines. The amides are reported to be useful as friction reducing agents when incorporated into fuels or lubricants. U.S. Pat. No. 4,406,802 describes various borated additive compositions which are useful in lubricating compositions as multi-functional additive compounds having friction-reducing and oxidation and corrosion-inhibiting characteristics. In general, the boron-containing compounds are borates of mixed alcohols, amides, amines, and hydroxy esters, ethoxylated amines and ethoxylated amides and mixtures thereof.